This invention pertains to lenses. In particular, this invention pertains to lenses that can be used to focus a laser beam on a small spot, or on two spatially separated small spots, but simultaneously provide large depth of focus. Such a lens is particularly useful in optical and magneto-optical disk drives, and drives that read and write to different disks with different overcoat thicknesses.
Referring to FIG. 1A, a prior art optical or magneto-optic disk drive typically comprises a laser source 1 for providing a laser beam 2, an optical or magneto-optic data recording disk 3, and a lens 4 for focussing the laser on a small spot on disk 3. A motor 5 rotates disk 3 so that laser beam 2 can scan over a data recording track of the disk. In such applications, the depth of focus is an important parameter which is preferably maximized. In conventionally designed optics, the depth of focus D is given by the following relationship: EQU D=0.8.lambda./(NA).sup.2
where NA is the numerical aperture of the lens and .lambda. is the laser light wavelength.
In optical and magneto-optic data storage, one also generally strives for high data recording density. This means that the spot size produced by the focussing lens should be as small as possible. To reduce the spot size of a given design, one increases the NA of the lens. However, as can be seen from the above equation, this has the detrimental effect of reducing the depth of focus D. This detrimental effect is especially important in the case of drives in which the disk spins rapidly, e.g. 40.times. drives. (The nomenclature "40.times." is part of a numbering system which is well known in optical storage and refers to a rotation rate 40 times that of standard audio CD players. An audio CD player spins at 1.times..) Because disk 3 spins rapidly, it can sometimes wobble, e.g. moving in the direction of arrow A, causing disk 3 to move closer to or further from lens 4, and causing the laser spot to be out of focus. In such drives, a shallow depth of focus puts an unmanageable burden on the frequency response of the auto-focus system of the drive. Accordingly, it would be desirable to provide a lens with a relatively large depth of focus and a small spot size.
There are several types of optical and magneto-optic media known in the art. FIG. 1B shows a prior art laser beam 6 passing through a lens 7 to read data from a DVD disk 8. DVD disk 8 comprises a transparent substrate 9 over a recording layer 10. Typical transparent substrates 9 on DVD recording media have a thickness T1 of about 600 microns.
FIG. 1B' shows laser beam 6 passing through a lens 11 to read data from a CD disk 12. CD disk 12 comprises a transparent substrate 13 over a recording layer 14. Typical transparent substrates 13 on a CD disk have a thickness T2 of about 1200 microns. Thus, in order to read both CD disks and DVD disks, an optical disk drive must have two lenses--one for reading CD media and one for reading DVD media. Such drives typically include a mechanism for switching lenses. Such a mechanism is cumbersome and adds expense to the disk drive. Further, it is difficult to substitute one lens for the other while continuing to maintain precise spacing between the lens and the media. Accordingly, it would be desirable to provide a lens capable of reading data from (or writing data to) different types of recording media having different substrate thicknesses.